1. Field
The present disclosure relates to optical communication systems and, in particular, to receivers of optical communication signals as well as optical and electro-optical components thereof.
2. State of the Art
Phase-shift keying (PSK) is a digital modulation scheme used in optical communication that conveys data by changing, or modulating, the phase of a reference optical signal (the optical carrier wave). PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator (or receiver), which is designed specifically for the symbol-set used by the modulator (or transmitter), determines the phase of the received optical signal and maps it back to the symbol it represents, thus recovering the original data. One form of demodulation requires that the receiver be able to compare the phase of the received optical signal to an optical local oscillating signal whose phase tracks the phase of the reference optical signal. Such a receiver is commonly referred to as a coherent PSK optical receiver. The coherent PSK optical receiver typically employs an optical phase lock loop to synchronize the phase of the optical local oscillating signal to the phase of the reference optical signal used by the transmitter.
Constellation diagrams are commonly used to represent PSK schemes. The constellation diagram shows points in the complex plane where, in this context, the real and imaginary axes are termed the in-phase and quadrature axes respectively due to their 90° separation. Such a representation on perpendicular axes lends itself to straightforward implementation. Specifically, the constellation points are typically selected with uniform angular spacing around a circle. This gives maximum phase-separation between adjacent points and thus the best immunity to corruption. They are positioned on a circle so that they can all be transmitted with the same energy. In this way, the moduli of the complex numbers they represent will be the same and the amplitude of the phase-modulated carrier wave does not change.
Two common PSK schemes are binary phase-shift keying (BPSK or 2-PSK) which uses two phases separated by 180 degrees, and quadrature phase-shift keying (QPSK or 4-PSK) or which uses four phases separated by 90 degrees, although any number of phases may be used. Since the data to be conveyed are usually binary (or digital) in nature, the PSK scheme is usually designed with the number of constellation points being a power of 2. The higher-order PSK schemes are typically labeled with the number of phases used for the respective scheme. Thus, a PSK scheme employing 8 phases separated by 45 degrees is commonly referred to as 8-PSK, a PSK scheme employing 16 phases separated by 22.5 degrees is commonly referred to 16-PSK, and a generic PSK employing M phases separated by (360/M) degrees is commonly referred to as M-PSK or M-ary PSK.
For the coherent BPSK optical receiver, the phase-aligned local oscillating signal can be split and mixed in the optical domain by a network of 3-dB fiber couplers for supply to corresponding photodetectors as shown in FIG. 1. This configuration converts the received optical signal to baseband electrical signals that represents the in-phase component and quadrature component of the received optical signal. These two component signals can be processed in the electrical domain by a signal processor in order to determine the phase of the received optical signal and map it back to the symbol it represents, thus recovering the original data.